In nuclear power plants, electricity is generated by rotating a turbine with steam generated in a reactor or steam generator, and then the steam is cooled by sea water and the resulting condensed water (condensate) is treated in a condensate demineralizer using an ion exchange resin and recycled to the reactor or steam generator. In the condensate demineralizer, an ion exchange resin is used to remove sea water components flowing into the system or suspended corrosion products based on iron oxides produced from plant construction materials or ionic impurities. The resin used in the condensate demineralizer is a combination of an anion exchange resin and a cation exchange resin (sometimes briefly referred to as “anion resin” and “cation resin”, respectively hereinafter), normally a combination of a gel-type cation resin and a gel-type anion resin or a combination of a porous cation resin and a porous anion resin.
Gel-type resins and porous resins have relative disadvantages of low osmotic resistance and low wear resistance, respectively. In view of these disadvantages, gel-type resins are normally used in condensate demineralizers in plants involving frequent backwash regeneration, while porous resins are used in plants involving frequent chemical regeneration. Especially, porous resins are poor in wear resistance so that they suffer from surface damage or breakage of resin particles by contacts between resin particles or between resins and metal materials of piping during transfer between demineralization columns and regeneration columns. For this reason, a gel-type cation resin and a gel-type anion resin with high wear resistance are used in plants involving backwashes to remove suspended corrosion products called clad deposited on cation resin surfaces such as nuclear power plants with boiling water type reactor. In addition, porous resins have a relatively dense resin matrix structure as compared with gel-type resins so that the diffusion speed of ions into pores of the resins during adsorption of the ions and the diffusion speed of ions adsorbed to the resins into washing water during chemical regeneration are lower than those of gel-type resins, resulting in lower performance in kinetics and regeneration efficiency. Thus, condensate demineralizers using porous resins must be designed to increase the regeneration level (the amount of the chemical used) or otherwise allow for characteristics of porous resins.
Ion exchange resins are chemically regenerated at the point when they have been consumed to a certain extent because their ion exchange capacity gradually decreases with the increase of ion load as water passes through them. This step normally comprises separation by upflow backwash taking advantage of the gravity difference between cation resins and anion resins. In order to increase the efficiency of separation by this operation, gel-type ion exchange resins having a uniform particle size distribution are commercially available and widely used in condensate demineralizers.
Ion exchange resins used in condensate demineralizers in nuclear power plants have high capacity to remove ionic components such as sea water components as typified by NaCl coming from upstream, but organic impurities (hereinafter referred to as TOC) leaching from cation resins are carried into the reactor or steam generator where they degrade to produce sulfate ions leading to water quality deterioration. Thus, the leakage of TOC leaching from ion exchange resins must be decreased to achieve high-purity effluent quality.
Methods for solving these problems have been proposed by e.g., adopting a strongly acidic gel-type cation resin having a crosslinking degree of 12-16%, which is higher than conventional degrees of 8-10% (JP-A HEI 11-352283), or adsorbing TOC leaching from cation resins to an anion resin placed in a lower (downstream) resin layer (JP-A 2001-314855), or forming a mixed bed of a strongly acidic gel-type cation resin and a porous anion resin having a Gaussian particle size distribution (JP-A HEI 8-224579).